Process control systems require the accurate measurement of process variables. Typically, a primary element senses the value of a process variable and a transmitter develops an output having a value that varies as a function of the process variable. For example, a level transmitter includes a primary element for sensing level and a circuit for developing an electrical signal proportional to sensed level.
Knowledge of level in industrial process tanks or vessels has long been required for safe and cost-effective operation of plants. Many technologies exist for making level measurements. These include buoyancy, capacitance, ultrasonic and microwave radar, to name a few. Some level measurement instruments measure the distance from a known location to a material surface. Particularly, distance measuring devices including those employing guided wave radar and through air radar technology often make use of a reference signal, referred to as a fiducial. The fiducial is a known location, typically in the instrument housing or at the top of a probe or antenna. The location of the fiducial must be determined accurately in order for the distance measurement to be accurate. The distance to the surface of interest is determined by the apparent difference in time of signals between the fiducial and the surface of interest.
Such technologies make use of an interface circuit comprising a drive circuit for transmitting a pulse signal at the fiducial and at the target of interest and a receive circuit to receive reflected echoes of the pulse signal. The typical echo for the fiducial signal is a pulse. The fiducial location is typically determined using a comparator that detects the leading edge of the fiducial echo signal at the point where it exceeds a predetermined threshold value. However, the edges of the fiducial signal are sloped. Changes in signal amplitude can occur due to drifting, electronic component characteristics, or temperature induced changes to the device's mechanical properties. Such changes in signal amplitude can result in an apparent change of the fiducial location. This is generally illustrated in FIG. 3. This will introduce measurement error.
The present invention is directed to solving one or more of the problems discussed above in a novel and simple manner.